1. Field of the Invention
The invention relates to a solid-state imaging device, a method of driving the solid-state imaging device and an imaging apparatus.
2. Description of the Related Art
In recent years, in a CCD (Charge Coupled Device) image sensor and an amplification-type image sensor which are known as solid-state imaging devices suitable for applying to video cameras, digital still cameras and the like, miniaturization in pixel size is proceeding by increase in number of pixels at high sensibility or reduction in image size. On the other hand, generally, the solid-state imaging devices such as the CCD image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor tend to be used in various environments such as indoor and outdoor, daytime and nighttime, therefore, an electronic shutter operation or the like is often necessary, in which exposure time is adjusted by controlling a charge storage period in a photoelectric conversion element according to variation of outside light and the like to make the sensibility be the optimal value.
As a method of expanding the dynamic range of the CMOS image sensor, a method of adjusting exposure time by releasing the electronic shutter at high speed, a method of taking plural frames at high speed and superimposing them, a method of allowing a photoelectric conversion characteristic at a light-receiving region to be logarithmic response and the like are known.
However, when using the method of releasing the electronic shutter at high speed in a picture-taking scene with high-contrast where bright areas and dark areas are mixed, it is difficult to secure sufficient exposure time especially in the dark area, namely, in a low luminance scene, therefore, S/N deteriorates and image quality is lowered. In the method of taking plural frames at high speed and superimposing them, S/N can be improved by superimposing images as compared to the method of simply releasing the electronic shutter, however, noise by readout is accumulated by the number of times of readouts corresponding to plural frames, therefore, S/N also deteriorates at the low luminance scene.
It is efficient that the dynamic range is expanded by the logarithmic response characteristic, however, fixed pattern noise caused by threshold variations of transistors operating in a subthreshold region becomes prominent especially at the low luminance area. For example, when photographing a person by the window from the room, if the sensibility is adjusted to the person, a scene of the window is saturated white and difficult to be reproduced. If the sensibility is adjusted to the scene of the window, the person is taken to be dark, S/N lowers because it is difficult to sufficiently secure a signal level and to obtain high-quality images even by the amplification after photographing.
In a photographing scene, it is necessary to realize high S/N by long exposure time in pixels with a small amount of incident light on the image sensor, and to expand the dynamic range by avoiding saturation in pixels with a large amount of incident light.
In related arts, as a method of realizing high S/N which is almost equivalent to the normal operation in pixels with low luminance, and expanding the dynamic range in pixels with high luminance, a technique written in IEEE International Solid-State Circuits Conference (ISSCC) 2005, pp. 354, February 2005 (non-patent document 1) is known. Specifically, as shown in FIG. 40, in an amplification-type image sensor in which a pixel 100 is arranged in a matrix form, which includes a photodiode 101, a transfer transistor 102, a reset transistor 103, an amplification transistor 104, and a selection transistor 105, when the transfer transistor 102 is turned off, if electrons are stored exceeding a certain level, a voltage to be applied to a control electrode is set to a level Vtrg, not a level making the transistor completely off as usual, in which the excess is allowed to be overflowed into a FD region 106.
When electrons are stored in the photodiode 101 and exceed the level Vtrg, leak to the FD region 106 is started in the subthreshold region. Since the leak is operated in the subthreshold region, the number of electrons remained in the photodiode 101 is a logarithmic response.
As shown in FIG. 41, after a reset operation at a period t0, storing is executed while the voltage Vtrg is applied to the control electrode of the transfer transistor 102. In a state of a period t1 in which the number of stored electrons is small, all electrons are stored in the photodiode 101, however, when the number of stored electrons exceeds the level of Vtrg, electrons starts leaking to the FD region 106 as shown at a period t2.
Since electrons leak in the subthreshold region, electrons are stored with the logarithmic characteristic with respect to the incident light intensity even when the storing is continued (t3). At a period t4, electrons overflowed in the FD region 106 are reset, and all electrons stored in the photodiode 101 are read out by a complete transfer. Relation between the incident light intensity and the number of output electrons is shown in FIG. 42. In the case of incident light having intensity exceeding the upper limit Qlinear of a linear region set by the voltage Vtrg, the number of output electrons is determined with the logarithmic response.
However, though it is reported that a dynamic range of 124 dB has been realized in the related art written in the non-patent document 1, the saturation level of the linear region in which high S/N is realized is less than half of a normal saturation level Qs. In addition, though the extremely wide dynamic range is realized with the logarithmic response, a logarithmic response circuit tends to be affected by threshold variations and the like, therefore, large fixed pattern noise remains in the wide dynamic range region, which is 5 mV in the logarithmic region when the fixed pattern noise in the linear region is 0.8 mV, even after a cancel operation for threshold variations is performed.
Accordingly, it is desirable to provide a solid-state imaging device, a method of driving the solid-state imaging device and an imaging apparatus, in which signal acquisition with linear and high S/N is possible without narrowing the normal saturation level at low luminance, at the same time, the dynamic range can be expanded while realizing good S/N in a linear region also with respect to incident light larger than the normal saturation level.